Advanced imaging

week 2 lecture

Cost and Radiation Compairisons

conventional radiographs = $450 and 1.5 mSv

CT = 2k and 6.0 mSv

MRI = 3.5k and no radiation

bone scan, whole skeleton = 1.4k and 6.3 mSv

CT Overview

Computed Tomography

creates images using multiple x-rays in cross-sectional (axial) slices from different projection angles

the beam rotates around the body, multiple projections pass through and an electronic detector array records the patterns of densities

CT scanner

motorized table moves the pt through a circular opening

x-ray source and detector assembly within the system rotate around the pt

• one rotation is ab 1s

• during rotation x-ray source produces a narrow, fan-shaped beam of x-rays that passes through a section of pt body

scout image

2d digital radiograph produced by CT scanner

used to localize the structures to be scanned

• orientation and good for planning purposes

CT planes

coronal, axial, sagittal

same for MRI

CT image

reflects tissue radiodensity without superimposition of other tissues

some loss of resolution due to volume averaging when there is an image with tissues that lie proximate or over each other

viewer sees several images/slices

images can be windowed to reduce the range of radiodensities displayed and focus on a particular tissue

Slice thickness

• is anatomy dependent

• can range from .5-2 mm for small joints and 2-3 mm for large

• thinner slices means image volume is smaller

  • less radiodensity and increased noise

  • greater radiation to produce the same image quality

Additional Forms of CT

3d CT - multiplanar reconstruction

CT myelogram - injection of contrast material into the spinal fluid

Cone Beam CT - often used in dental practice - single volume of data means shorter scanning time and less radiation

CT benefits

best for identifying subtle fractures and/or complex fractures

best for evaluating degenerative changes

first choice for trauma - images osseous and soft tissues structures

excels in evaluation of spinal stenosis - especially if performed as a CT myelography

combined with diskogram = gives condition of intervertebral disk

best modality for evaluation of loose bodies in a joint

less time consuming (MRI and ultrasound) and less expensive (MRI)

allows for accurate measurements of osseous alignment in any plane

less problematic for pt with claustrophobia

CT limitations

limits the exam based on radiodensity

radiation exposure

CT roughly 4x single radiograph dose

CT in other medical specialties

Neuroimaging - best modality for acute settings, and a ‘head CT’ is standard protocol in trauma for immediate assessment of intracranial bleeding

cardiac imaging - CT angiogram

Pulmonary imaging

CT report

levels imaged

contiguous or interrupted slices

slice thickness

reformatting/reconstructions

angulation of gantry

windows provided

use of contrast agent

MRI overview

no ionizing radiation

processes to create image:

• signal generation based on the properties of magnetic resonance

• relaxation processes

• signal detection

• encoding of spatial information

• reconstruction of the image from the signal

• manipulation of tissue-dependent contrast

MRI Hardwar schematic

magnet

gradient coils

RF coils - to transmit and receive

MRI science

hydrogen atoms in the body have magnetic moment

external magnets produce resonance from each proton in the hydrogen atom

machine detects proton’s position/signal and takes a ‘photo’ of the hydrogen molecules in the body and then digitizes it into an image of the body part

T1 weighted = captures early signal decay

T2 weighted = captures late stage of signal decay

before imaging - hydrogen protons are aligned in random directions

superconducting magnet creates a strong magnetic field aligning protons in same directon

radiowaves are transmitted through body and the waves jostles the protons a bit off their axis and spin them in the same direction

radiowaves = turned off, protons return to aligned positions; time it takes ions to return to alignmen is measured by scanner → different issue shave a unique time frame

computer uses data to assemble a detailed image of body

T1 weighted image

bone marrow gives rise to relatively high signal intensity

CSP gives rise to low signal intensity = dark

T2 weighted image

CSF shows high signal intensity

MRI protocols

sequence for MRI - not standard like in radiography

spin-echo (SE): T1 and T2 imaging

Gradient-echo (GRE) sequences

• fast image acquisition

• high resolution and thin slices

• high contrast between fluid and cartilage - depends on parameters

MRI Open Scanner Advantages

greater ability to scan claustrophobic or obese pt

reduction of scanning noise

possibilities of performing tests or procedures during scanning

MRI Open Scanner disadvantages

lower field strength → requiring adjustments of imaging sequences

lower signal-to-noise ratio

longer scanning times

can be more fuzziness due to more pt movement

MRI upright scanner advantages

ability to examine the spine under weight-bearing conditions → spine looks different than while sitting or laying down

ability to scan pt. who are to large to fit into the bore of the magnet or must be scanned in the upright position becuase of conditions such as CHF or severe thoracic kyphosis

MRI upright scanner disadvantages

longer scanning times (3x than conventional high-field scanner)

possible image degradation due to longer scanning times and lower field strength

placement of pt in a painful position which may lead to increased pt. movement during examination and degraded image quality

MRI clinical use

good at finding changes and variations in bone marrow

• differential diagnosis of bone tumors, stress fractures, and avascular necrosis

good soft tissue detail - tendons, ligaments tears, meniscal tears

best modality for evaluation of disk herniations and other potential causes of nerve root involvement

ability to stage neoplasms in bone and soft tissues as well as evaluate extent of tissue invasion prior (and after) to surgery